1. Field of the Invention
This invention relates generally to means and techniques for measuring the flow rate of a water stream in an open channel.
More particularly, this invention is directed to a weir box and weir, a water level sensor and associated structures to measure the rate of, and to document the volume of, water flow.
A specific embodiment of this invention is directed to the measurement of water flow that exits a pressurized pipe, enters a containment means such as a weir box, and then flows over a weir at atmospheric pressure.
2. Description of Related Art
A number of standard techniques and devices have been developed and are in use for the measurement of water flow rates within an open channel. Those include weirs, Parshall flumes, current meter gauging stations and the like. Almost anything that partially restricts flow in a ditch or channel can be used as a measuring device if it is calibrated. A calibrated restriction for measuring water flow may be broadly classified as either a flume or a weir. A flume comprises a structure that defines a specially shaped flow section that either provides a restriction in channel area or a change in channel slope, or both. Flow rate is determined by measuring the water depth at a specified location within the flume.
Probably the most common flow measurement method, and the method to which this invention is directed, employs a weir which is a calibrated restriction that controls the shape and velocity of the water flow passing the restriction. The flow rate is then determined by measuring the vertical distance from the apex or vertex of the overflow portion of the weir to the water surface near, but upstream from, the restriction. Calibration procedures are then used to develop equations or mathematical relationships that covert the water surface measurement to a flow rate. Flow volume is determined by integrating the flow rate over time.
There is an increasing need for the monitoring of flow rates and flow volumes from a variety of industrial, agricultural, and extractive activities. Those activities include the monitoring of industrial waste water outfalls, the rate and volume usage of irrigation water, and the dumping of by-product water from mining activities and from oil and natural gas production. A need for monitoring water flows is particularly acute in the production of coal bed methane as the coal beds are also aquifers, and much water is produced in the recovery of the natural gas.
Another factor that complicates the routine monitoring of water flows is that many of the metering locations are widely dispersed and remote and are subject to weather extremes, particularly extremely cold weather that often causes freezing of the water in the monitoring structure. In most instances, the height of the water surface relative to a weir reference point is determined through use of a pressure transducer that is placed at a fixed location relative to the weir reference point. Transducer pressure readings are thus translated directly into water levels. Freezing of the water in which a transducer is placed will destroy the transducer, and transducers that are accurate and stable enough for use are quite expensive.
The invention described in this application provides a new and practical way to monitor water flows, especially those flows that make a transition from pressurized pipe flow to atmospheric pressure in a weir box or other containment means, using a weir structure and a pressure transducer while at the same time avoiding damage to the transducer from freezing.